In increasingly wide spread applications of integrated analogue circuitry such as constant voltage generating, and voltage detecting circuits, the adjustment of generated voltages or detected voltages with high precision is of great importance.
In order to provide such adjustments to the conditions of the circuitry, laser beam trimming techniques are utilized after completing circuit fabrication such that the resistors and fuse elements are irradiated with a laser beam to be subjected to the resistor disconnection or characteristic change through the irradiation.
Among various types of the resistors, thin film metal resistors are attracting much attention for their low thermal coefficients of the resistance value (TCR).
Suitable examples of the materials for forming the resistors include chromium silicon (CrSi), nickel chromium (NiCr), tantalum nitride (TaN), chromium silicide (CrSi2), chromium nitrogen silicide (CrSiN), and chromium silicon oxide (CrSiO).
By irradiating directly with a laser beam, the thin film metal resistors undergo the resistor disconnection or characteristic change, and resistance values of the resistors can be adjusted.
Moreover, it is feasible to make the laser beam scanned over the area of the thin film metal resistors and to perform resistance adjustment (trimming) while measuring voltage outputs, for example, of fabricated circuit (which is generally called as on-line trimming). The on-line trimming of the thin film metal resistors yields an advantage of achieving a precision adjustment over a wide range of the resistance.
The adjustment of circuit parameters of the constant voltage generating circuit and voltage detecting circuit is carried out in general by properly adjusting a resistance ratio of a dividing resistance circuit included in the abovementioned circuits.
For example, as to the dividing resistance circuit formed of thin film metal resistors, a known resistance circuit is formed including a ladder trimming resistor 31 and an analog trimming resistor 33 in combination as illustrated in FIG. 10.
Thereafter, by irradiating the resistance circuit with a laser beam along a first beam locus 35a as shown in FIG. 10 to cause a resistor disconnection or characteristic change over an arbitrary number of the thin film metal resistor 33, a coarse adjustment is carried out of resistance value.
In the next place, by further irradiating the resistance circuit along a second beam locus 35b to cause a resistor disconnection or characteristic change over arbitrary regions on the thin film metal resistor 33, a desired series resistance value can be obtained with high precision (For example, Japanese Laid-Open Patent Application No. 8-124729).
In the resistance circuit formed of resistors with the ladder type arrangement as shown in FIG. 10, the resistance adjustment is carried out in general by performing (1) a coarse adjustment by trimming the ladder trimming resistor 31 as illustrated in FIG. 11A, and subsequently (2) a high precision adjustment over a partial region by trimming the analog trimming resistor 33 as illustrated in FIG. 11B.
In the trimming of the analog trimming resistor 33, however, the range of resistance subjected to the adjustment is relatively narrow.
As a result, several difficulties may be encountered in that the high precision adjustment cannot be feasible by the analogue trimming continuously over the regions covered by several coarse adjustments without the increases in (1) the number of the thin film metal resistors 31a in the ladder trimming resistor 31, (2) the area of resistor layout, or (3) trimming time.
In addition, another dividing resistance circuit is disclosed, as illustrated in FIG. 12, including an Rbottom resistor, (m+1) resistors SB0, SB1, . . . , SBm, with m being a positive integer, and an Rtop resistor, which are connected in series. In addition, the resistors SB0, SB1, . . . , SBm are respectively accompanied by corresponding fuse elements (or fuses) RL0, RL1, . . . , RLm connected in parallel with respect to the resistors, respectively, (For example, Japanese Laid-Open Patent Application No. 2003-37179).
In such dividing resistance circuit, a desired series resistance can be obtained by disconnecting arbitrary fuse elements RL0, RL1, - - - , RLm by a laser beam irradiation, for example (which is called conventionally as the digital trimming).
In the dividing resistance circuit of FIG. 12, it is possible to obtain a wide range of resistance adjustment by constituting the resistors each having binary resistance values (such as R, R/2, R/4, etc).
FIGS. 13A and 13B are graphical plots of resistance values as a function of the number of trimming step in the digital trimming for the dividing resistance circuit.
FIG. 13A plots the overall change in resistance values with the number of trimming steps, while FIG. 13B shows an expanded drawing for the portion of FIG. 13A ranging from the trimming steps 390 to 410 comparing the values obtained in practice with those by computation.
In the addition, the ten-bit code ranging from one through 1024 is adopted to the trimming steps of FIGS. 13A and 13B.
It should be noted that the digital trimming is performed such that the resistance values are measured only after completing the wafer fabrication and that the trimming of fuses are carried out according to the results of the resistance measurement, that is, the resistance adjustment through the off-line trimming.
As a result, the accuracy of the voltage output is considerably affected by the scattering in resistor characteristics caused by process fluctuation (FIG. 13B) This has given rise to another difficulty in achieving a high precision trimming of the dividing resistance circuit.